Pseudomonas fluorescens strain CHA0 suppresses various plant diseases caused by soilborne fungi and produces several antibiotic metabolites, two of which have been identified as pyoluteorin (Plt) and 2,4-diacetyl-phloroglucinol (Phl). A cosmid (pME3090) carrying a 22-kb insert of strain CHA0 DNA enhanced, in a CHA0 background, the production of Plt in vitro. The production of Phl was also increased but to a smaller extent. In a gnotobiotic system, the recombinant strain protected cucumber plants against disease caused by Pythium ultimum more than did wild type CHA0. In the absence of the pathogen, strains CHA0 and CHA0/pME3090 had no influence on cucumber. Both strains protected cress and sweet corn from P. ultimum. However, in the presence of P. ultimum, fresh weights of cress and sweet corn plants protected by strain CHA0/pME3090 were lower compared to those of plants protected by strain CHA0. Strain CHA0/pME3090 (but not strain CHA0) strongly reduced the growth of cress and sweet corn in the absence of the pathogen. In vitro, Plt was much more toxic to the growth of P. ultimum than was Phl. Sweet corn, cress, and cucumber were all inhibited by Plt and Phl; cucumber, however, was less sensitive to Phl than were cress and sweet corn. These results suggest that enhanced pyoluteorin production might be responsible for the increased capacity of strain CHA0/pME3090 to suppress damping-off of cucumber and that the strain's deleterious effect on cress and sweet corn might be due to the phytotoxic properties of both antibiotics. We conclude that, depending on the host-pathogen system, enhanced antibiotic production by P. fluorescens may result in improved disease suppression or, in contrast, in a toxic effect on the plant.